The proposed research is directed towards understanding the molecular mechanisms regulating gene expression in a simple eukaryotic organism, the yeast Saccharomyces cerevisiae. In particular, we propose to continue our studies of a number of aspects of the yeast mating process. The mating process is controlled by the mating type locus, which can exist in one of two states, a or "alpha". These alleles control the production of and response to mating-type specific pheromones, alpha factor, a factor, and a new pheromone discovered in our laboratory. We propose to determine the manner in which the mating type locus regulates such mating specific functions by biochemical and genetic analysis of presently-existing mutants defective in mating, and by analysis of mating-deficient mutants isolated by new techniques. In addition, we propose to isolate mutants in which control of these factors by the mating type locus is by-passed. Although mating types a and alpha are stable to growth, rare interconversions from a to alpha and from alpha to a can be observed spontaneously. In addition, a gene (HO) exists which promotes this interconversion at high frequency. We propose to study the mechanism by which the mating type switch occurs. Specifically, we propose to isolate and characterize mutants defective in the ability to interconvert mating types. In addition, we shall investigate the mechanism by which function is restored to a defective mating type locus by action of the HO gene. This study of control of the mating process in yeast will provide information concerning the molecular bases by which genes are controlled and by which a stable switch is established and maintained in a eukaryotic organism.